1. Field of the Invention
The present invention relates generally to high NA optical fibers, including fibers with high NA large cores for short distance communication systems, or with high NA claddings in double clad fibers for use with high power light sources or in optical fiber lasers and optical amplifiers.
2. Technical Background
Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. Optical fibers have also been utilized in automotive applications because they offer high bandwidth, are relatively inexpensive, and can be used in automotive optical data systems to provide optical data for information system, entertainment, engine management and safety systems. Such fibers require high numerical aperture of the fiber core and a large core, to provide for efficient coupling of light into the fiber core. Two types of optical fiber are now utilized in automotive applications. They are polymer optical fibers (POFs) and polymer cladded silica (PCS) optical fiber. It is the relatively low index of refraction of polymer that provides a high core NA. The main draw back of POF fiber is its relatively high attenuation at the wavelength of interest (as high as 0.3 dB/m to 0.4 dB/m for 630 nm<λ<685 nm) and relatively narrow temperature range of operation (−45° C. to 85° C.). For applications such as sensor systems for safety, engine management system it is preferable to have wider operating fiber temperature range, while applications such as video processing systems require a fiber with better attenuation. PCS fiber can operate in a somewhat broader temperature ranges (−65° C. to 125° C.), and have a lower attenuation than the POF fiber. However, at higher temperatures, PCS of optical fiber heats and the polymer material of the outer cladding layer carbonizes or burns, resulting in device failure, especially when the fiber is bent. Even at the temperatures between 85° C. and 125° C., the polymer cladding ages relatively quickly, losing its mechanical and optical characteristics and becoming brittle, thus shortening the device life. Finally, both of these types of fiber suffer from bend losses.
Single clad rare earth doped optical fiber has been widely used in the field of optical amplifiers and fiber lasers. This type of fiber has low capability of handling high power multimode optical sources due to the difficulty of efficiently coupling multimode light from a high power optical (light) source (also referred to herein as optical pump or pump) into the rare-earth doped fiber core.
To solve this problem and to increase the output power of fiber lasers, those of skill in the art utilize optical fiber with a double clad structure (referred herein as double clad optical fiber). Double clad rare-earth doped optical fiber is a fiber that has a core, an inner cladding layer surrounding the core and an outer cladding layer surrounding the inner cladding layer.
Double clad optical fiber has been used in applications requiring utilization of optical sources providing between 10 to 100 Watts of optical power, because double clad optical fiber is more efficient than single clad optical fiber in retaining and in utilizing optical power provided by the pump. This higher efficiency is due to fiber's utilization of inner clad-to-core coupling of optical pump power. More specifically, rare-earth doped double clad optical fibers accept light from the optical pump into the inner cladding and then transfer light to the rare-earth doped core through the core-to-inner cladding interface, along the length of the optical fiber. Thus, the optical fiber converts a significant part of the multi-mode light propagated through the inner cladding into a single-mode output at a longer wavelength, by coupling this pump light into the rare-earth doped core.
The inner cladding of the double clad optical fiber has a higher index of refraction than the outer cladding, thus the pump energy is confined inside the inner cladding and is re-directed into the core. The optical fiber is optically active due to the presence of rare-earth dopant in the core, which can be excited to higher electronic energy levels when the optical fiber is pumped by a strong optical pump. Cladding pumping can be utilized in fiber amplifiers, or employed to build high-power single mode fiber pump lasers.
In a double-clad laser, an outer cladding of the optical fiber confines the pump light provided by an optical pump in the optical fiber's multi-mode inner cladding. The much smaller cross-sectional area of the optical fiber's core is typically doped with at least one rare-earth element, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. The double-clad arrangement facilitates pumping of the fiber using a multi-mode first cladding for accepting and transferring pump energy to a core along the length of the device.
How much pump light can be coupled into a double-clad fiber's inner cladding depends on the cladding size and numerical aperture NA. Typically, a high numerical aperture NA of the inner cladding, which is related to the difference in refractive index between the inner and outer cladding, is desired. In the well-known design, the first clad layer (inner cladding) is made of glass and the second layer (outer cladding) is made of plastic (for example, fluorinated polymer) with relatively low refractive index in order to increase the numerical aperture NA of the inner cladding. Such plastic may not have the desired thermal stability for many applications, may delaminate from the first cladding, and may be susceptible to moisture damage. In addition, this type of double clad optical fiber may be suitable only for sustained use with relatively low power (lower than 20 Watts) optical sources. When high power sources (more than 100 Watts) are utilized, this type of optical fiber heats and the polymer material of the outer cladding layer carbonizes or burns, resulting in device failure, especially when the fiber is bent. At medium powers (20 Watts to below 100 Watts), the polymer outer cladding ages relatively quickly, losing its mechanical and optical characteristics and becoming brittle, thus shortening the device life.